1. Field of the Invention
The invention relates generally to the field of spatial-heterodyne interferometry (SHI). More particularly, the invention relates to methods and machinery for obtaining spatial-heterodyne interferometry for transmission (SHIFT) and spatial-heterodyne interferometry for reflection and transmission (SHIRT) measurements.
2. Discussion of the Related Art
U.S. Pat. Nos. 6,078,392 and 6,525,821 relate to Direct-to-Digital Holography (DDH). In DDH, a reflected object wavefront is combined with a reference wavefront at a small angle on the surface of a digital imaging device. The small angle generates a set of linear fringes that spatially heterodynes the reflected object wavefront. Fourier analysis is then used to isolate the image at the heterodyne frequency and reconstruct the complex wavefront, Voelkl (1999).
DDH is an implementation of spatial-heterodyne interferometry with Fourier reconstruction to capture complex wavefronts reflected from the surface of an object. When a wavefront strikes the surface of an object, the shape of the surface is imbedded in the phase of the wavefront and the reflectivities of the surface are contained in the intensity of the reflected wave. This reflected wave is combined with a reference wave at the digital imaging device so that they interfere and create a set of linear interference fringes. These linear interference fringes then contain the phase and amplitude information of the object wave. In Fourier space, this object wave information shows up centered around the spatial-frequency of the fringes. The recording of a wave's phase and amplitude information at a nonzero frequency is known as “heterodyning.”
However, DDH does not provide information regarding the interior of an object of interest and only provides information regarding the surface of the object.
Meanwhile, phase contrast microscopy (PCM) is a well known technique that is commonly used to image biological specimens. PCM is particularly useful when a biological sample includes phase differentiable features that are of similar transmissibility. However, a limitation of PCM is that no complex wavefront information is provided, the phase information from PCM being represented by amplitude only.
Recently, Jacob (2002) reported a technique in which transmission phase shift interferometry is used to measure phase differences between two points on a photolithographic mask. While this technique is able to measure phase changes, 30 seconds are required for each height measurement and, therefore, a slow scan would be required to measure phase changes across an interior portion of an object and a very long scan would be required to measure phase changes over an entire mask. Therefore, what is needed is an approach that can quickly provide complex wavefront information regarding an interior portion of an object.
Heretofore, the requirements of both providing complex wavefront information regarding an interior portion of an object and providing that information quickly have not been met. What is needed is a solution that simultaneously solves both of these problems.